Although the physics of RFID apply to all frequencies, there are, in fact, only a few frequency ranges that RFID systems are allowed to use without requiring specific, expensive spectrum licenses. The recent 3G spectrum auction in the United States resulted in a total sale price of several billion dollars. To avoid these expensive, proprietary licenses, all commercial RFID systems are designed to work in the free-to-use but highly regulated Industrial, Scientific, and Medical (ISM) bands. These frequency bands are reserved for so-called short-range devices by national and international organizations and governments.
The rules and regulations that define what frequencies can be used by RFID systems also define the allowed bandwidth, radiation powers, transmission times, modulation modes, and other operation modes for the devices. Although the International Telecommunication Union (ITU) encourages worldwide harmonization of these rules and regulations, not all frequency ranges are usable worldwide. This is significant because tags that must operate within multiple-frequency bands are either more expensive or have a shorter maximum communication range than tags that need to operate within only a single narrow frequency band.
Why are there different frequency bands? The answer is simple: different frequencies have different propagation characteristics. All frequencies are attenuated and reflected by materials to a greater or lesser degree, with the higher frequencies being more greatly attenuated than the lower frequencies. Low frequencies, such as the 125 kHz frequency, are attenuated very little as they propagate through materials. This allows them to have significant signal-penetration capabilities through all materials including metal. When radiated and used in the far field, these frequencies can also have a significant communication range. Those of us who have traveled across the United States by car at night have experienced the propagation wonders of these low frequencies as we listened to AM radio stations (typically operating between 580 kHz and 1700 kHz) that were being broadcast 100 miles or more away from us. Try doing that with an FM radio station. (FM radio stations typically operate between 88 MHz and 108 MHz.)
Ultra-high frequencies, such as the 915 MHz frequency, are highly attenuated and reflected by most materials. This limits, for example, the depth of water through which a UHF tag may be read and the distance at which a reader may be heard. This also means that electromagnetic reflective surfaces, such as metals, act as very good mirrors for UHF energy incident upon them. Even cardboard will reflect some of the UHF energy incident upon it.
All materials, including air, attenuate magnetic fields as the fields propagate through the material. The relative ease with which a magnetic field propagates through a material is dependent on the material's permeability. Permeability is a material property that describes the ease with which a magnetic flux is established within the material. Permeability, μ, is the ratio of the magnetic flux density (B) to the magnetic field (H) creating the flux (often referred to as the magnetizing force), μ = B/H. The permeability of air is 1.256 × 10-6 H/m (Hertz per meter).
The depth of penetration of a magnetic field through a material is inversely proportional to the square root of the product of the frequency and the permeability of the material. Consequently, the higher the frequency of the signal generated at the antenna, the lower the depth of penetration through a specific material. The net result is that aluminum acts as a better shield against magnetic energy than does copper or steel.
The term gain seems to cause some confusion but it's really quite simple. Think of a reader as radiating a fixed reference amount of power, an antenna with a higher gain can increase that output (measured in decibels [dB]). The reader reference is usually known as 0 dBD (zero decibels referenced to a dipole). Figuring out the antenna gain in dBD is simple if you have a scientific calculator or computer. The equation is:
Antenna gain (dBD) = 10*log (Power output/Power input)
If you have looked into rules of various governing bodies you will often see the acronym ERP used, which stands for effective radiated power from an individual antenna. Effective radiated power is quite simply the power supplied to an antenna times the antenna gain in a given direction, or as the product of the antennas power and its gain relative to a half-wave dipole in a given direction:
ERP (dBm) = Power in dBm - loss in transmission line (dB) + antenna gain in dBd